const Dates_NanosecondBased_Period = Union{
    Base.Dates.Nanosecond, Base.Dates.Microsecond,
    Base.Dates.Millisecond, Base.Dates.Second,
    Base.Dates.Minute, Base.Dates.Hour,
    Base.Dates.Day, Base.Dates.Week
    }

const Dates_MonthBased_Period = Union{Base.Dates.Month, Base.Dates.Year}

const NanosecondBased_Period = Union{Nanosecond, Microsecond, Millisecond,
                                     Second, Minute, Hour, Day, Week}
                   #                         Minute, QuarterHour, Hour, Day, Week}
                   # Decade, Century
const MonthBased_Period = Union{Month, Year}
#const MonthBased_Period = Union{Month, Quarter, Year}

const ContextualClocks = Union{UniversalTime, LocalTime, ZonedDateTime}
const SingleUnitDurations = Union{Period, Duration, Base.Dates.Period}
const DoubleUnitDurations = Union{TimeSpan, Base.Dates.CompoundPeriod}
